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Abstract

One-dimensional anodic titanium oxide (ATO) nanotube arrays hold great potential as
photoanode for photoelectrochemical (PEC) water splitting. In this work, we report
a facile and eco-friendly electrochemical hydrogenation method to modify the electronic
and PEC properties of ATO nanotube films. The hydrogenated ATO (ATO-H) electrodes
present a significantly improved photocurrent of 0.65 mA/cm2 in comparison with that of pristine ATO nanotubes (0.29 mA/cm2) recorded under air mass 1.5 global illumination. The incident photon-to-current
efficiency measurement suggests that the enhanced photocurrent of ATO-H nanotubes
is mainly ascribed to the improved photoactivity in the UV region. We propose that
the electrochemical hydrogenation induced surface oxygen vacancies contribute to the
substantially enhanced electrical conductivity and photoactivity.

Keywords:

Background

Continued research efforts over the past few decades on solar water splitting have
led to a substantial improvement in both scientific understanding and technical application
[1-4]. Because of its abundance, nontoxicity, and stability, TiO2 is one of the most promising photoanodes in the solar water splitting system. So
far the solar-to-hydrogen (STH) efficiency of TiO2-based photoanodes is limited by a wide bandgap (3.0 to 3.2 eV) and numerous electron–hole
recombination centers
[5]. A variety of approaches have been explored to enhance the visible light activity
of TiO2, such as metal doping
[6] or nonmetal doping
[7,8].

Recently, hydrogenation of TiO2, with intentionally introduced Ti3+ or oxygen vacancy states, has been proved to be an effective strategy for improving
the electronic conductivity and photoresponse property
[9-14]. Annealing processes in hydrogen atmosphere either under high temperature
[13,14] or by a long processing duration
[11] are two most employed ways. However, the need for either high-energy consumption
or expensive facility would limit its practical application. Alternatively, the electrochemical
reductive doping process provides another simpler approach for TiO2 hydrogenation. Under an external electric field, hydrogen is driven into the TiO2 lattice and reduces Ti4+ to Ti3+[15,16]. The intentionally introduced donor states associated with enhanced conductivity
have delivered a variety of applications in template synthesis
[17,18], electrochemical supercapacitors
[19], and photovoltaic devices
[20].

Methods

Ti foils (99.7%, 0.2 mm thickness, Shanghai Shangmu Technology Co. Ltd) were ultrasonically
cleaned in acetone, ethanol, and deionized water successively after an annealing process
(450°C for 2 h). Then electrochemical polish was carried out in a solution of acetic
acid and perchloric acid which determined the flat surface of the Ti foils. ATO nanotube
films were made by two-step anodization in ethylene glycol electrolyte containing
0.3 wt.% NH4F and 10 vol.% H2O. First-step anodization was performed at 150 V for 1 h in a conventional two-electrode
configuration with a carbon rod as cathode electrode. The as-anodized nanotube films
were removed from the Ti foil with adhesive tape
[20]. Second-step anodization was performed under the same condition for 1 h. The ATO
products were crystallized in ambient air at 150°C for 3 h, then up to 450°C for 5
h with a heating rate of 1°C/min.

The electrochemical reductive doping process was performed in a two-electrode system
at room temperature, employing the ATO nanotubes and a Pt electrode as the cathode
and anode, respectively, with the distance between the two electrodes exactly fixed
at 2.5 cm. The crystallized ATO nanotubes were immersed in 0.5 M Na2SO4 aqueous solution, and a voltage of 5 V was imposed between the electrodes. The reductive
doping duration was maintained in the range of 5 to 40 s, and the optimum time was
found to be 10 s. Finally, the ATO nanotubes were taken out, washed with deionized
water, and dried for measurements.

The PEC water splitting performances of the ATO nanotubes without and with electrochemical
hydrogenation were evaluated by AUTOLAB using a three-electrode configuration with
the nanotube films (1 × 1 cm2) as working electrode, Ag/AgCl (3 M KCl) electrode as reference electrode, and a
platinum foil as counter electrode. The supporting electrolyte was 1 M potassium hydroxide
(KOH, pH = 14) containing 1 wt.% of ethylene glycol solution, where ethylene glycol
acted as a potential hole scavenger (electron donor) to minimize the recombination
of charge carriers
[24]. The photocurrent was measured at a potential of 0 V (vs Ag/AgCl) under chopped light
irradiation with UV light (5.8 mW/cm2 at 365 nm) and simulated solar illumination (100 mW/cm2) from a Xe lamp coupled with an air mass 1.5 global (AM 1.5G) filter (Newport no.
94063A). The incident photon-to-current conversion efficiency (IPCE, DC mode) was
measured in three-electrode configuration by an AUTOLAB electrochemical station with
the assistance of a commercial spectral response system (QEX10, PV Measurements, Inc.,
Boulder, CO, USA). In order to record the stable photoresponse from photoanodes, each
wavelength was held for 3 min before the photocurrent measurements. Impedance measurements
were performed under dark condition at open-circuit potential over a frequency range
of 100 kHz to 0.1 Hz with an amplitude of 10 mV.

Results and discussion

Figure
1a represents the cross-sectional views of ATO film after second-step anodization in
which a vertically aligned one-dimensional feature is observed. The average outer
diameter of nanotubes is approximately 300 nm, with a tube wall thickness around 75
nm. Figure
1b shows the TEM image of two adjacent nanotubes with crystalline nanoparticles (D = 25 to 50 nm) arranged along the nanotubes. The XRD patterns of the ATO and ATO-H
nanotube films are shown in Figure
1c. Except for the peaks at 40.25°, 53.06°, and 70.71° that originated from the Ti
metal, all other peaks are coincident with each other and can be indexed to anatase
TiO2 (JCPDF no. 21–1272). The average crystallite size variation from 31.9 nm (ATO) to
31.3 nm (ATO-H), estimated from the major diffraction peak (2θ = 25.17°) using Scherrer's equation
[25], is less than 2%. After scraping the ATO nanotube powders off the Ti foil substrates
with a razor blade, a distinct color evolution is revealed from white (ATO powder)
to blue-black (ATO-H-10) (inset of Figure
1c). The evolution of optical properties could be ascribed to the increased defect
density
[11] on tube surface as disclosed by the Raman spectroscopy analysis.

Figure 1.The morphology and structure characterization of ATO and ATO-H. (a) A side view of ATO nanotube film after second-step anodization. Inset of (a) shows
an enlarged image indicating a smooth tube wall. (b) A TEM image of ATO nanotubes. (c) XRD patterns of pristine ATO and ATO-H-10 films. Inset of (c) shows the photographs
of ATO and ATO-H nanotube powders. (d) Raman spectra of the pristine ATO and ATO-H nanotubes with different processing time
(5, 10, and 30 s).

Figure
1d displays the Raman spectra of ATO nanotubes treated with different reductive processing
times (denoted as ATO-H-5, ATO-H-10, and ATO-H-30 for 5-, 10-, and 30-s treatments,
respectively). The six Raman vibrational mode of anatase TiO2 samples
[26] can be found at 148.4 cm-1 (Eg(1)), 200.5 cm-1 (Eg(2)), 399.1 cm-1 (B1g(1)), 641.2 cm-1 (Eg(3)), 520.6 cm-1 (A1g), and 519 cm-1 (B1g(2) superimposed with 520.6 cm-1), which is in agreement with the above XRD results. A slight blueshift and broadening
of Eg(1) and Eg(2) peaks are observed in the ATO-H-10 sample, suggesting increased surface disorder
due to the introduced oxygen vacancies
[10]. According to the above analysis, the possibly introduced defect states originate
from the formation of oxygen vacancies on ATO nanotubes.

The photocurrent densities of ATO-H photoanodes at a constant potential of 0 V (vs
Ag/AgCl) under the standard AM 1.5G solar light illumination are subsequently recorded
as a function of reductive doping duration with respect to pristine ATO electrode
(Figure
2a). Each duration is measured in at least three samples to average out the experimental
fluctuation. The photocurrent densities increase gradually with the processing time,
yielding a maximum value of 0.65 mA/cm2 for a 10-s treatment. Further prolonged processing time leads to a depressed performance,
which could be ascribed to increased surface defect density and corresponding recombination
rate. Thus, ATO-H electrodes with a 10-s doping duration (ATO-H-10) are employed in
the following experiments unless otherwise specified.

Figure
2b, and c show the photocurrent of ATO and ATO-H-10 under illuminations of chopped
UV (5.8 mW/cm2 at 365 nm) and simulated solar light (100 mW/cm2) at a constant potential of 0 V (vs Ag/AgCl). In comparison with the photocurrent
density generated on pristine ATO (0.25 mA/cm2 under UV irradiation and 0.29 mA/cm2 under solar irradiation), the ATO-H-10 electrode delivers a much improved performance
(0.56 mA under UV irradiation and 0.65 mA/cm2 under solar irradiation). Meanwhile, Figure
2d presents the chronoamperometric curves under simulated solar illumination for characterizing
the long-term stability of nanotube photoelectrodes. Both curves were kept stable
within the measurement period, indicating good stability after electrochemical hydrogenation.

Linear sweeps voltammetry (LSV) is a voltammetric method where the potential between
the working electrode and a reference electrode is linearly swept in time with simultaneously
recorded current. In the PEC water-splitting system, LSV is widely employed to characterize
the photoelectrodes’ performance with quantitative open circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and light-to-hydrogen efficiency. However, unlike most solid-state
solar cells, the linear sweeps in this liquid system are strongly dependent on the
scan rate
[27]. Under a fast potential scan, the thickness of diffusion layer will decrease from
the electrode in comparison with the one under a slow scan. Consequently, the ionic
flux towards electrode surface associated with current density will be increased.
Therefore, the scan rate is worthy of serious consideration in evaluating the electrode
performance. One could give an overestimated and misleading STH efficiency if an inappropriate
high scan rate was applied.

Figure
3a shows the LSV curves of ATO-H-10 measured as a function of scan rates. The photocurrent
densities are elevated within the entire potential window by increasing the scan rate.
A low scan rate of 5 mV/s is adapted in the following experiments, which will accommodate
better with the results in photocurrent transients. Figure
3b shows the LSV characteristics of ATO and ATO-H-10 nanotubes under simulated solar
illumination. The reductive doping process substantially improves the photocurrent
density almost in the whole potential window except for a slightly decrease of Voc. The positive shift of Voc indicates that the hydrogen-induced defects lead to a relatively faster recombination
rate as proven by TRPL measurements (shown below). It is worth noting that the Jsc (0.66 mA/cm2) across the ATO-H-10 electrode (with a scan rate of 5 mV/s) is even higher than the
highest photocurrent density (approximately 0.63 mA/cm2) ever reported on hydrogenated ATO nanotubes obtained from high-temperature annealing
in hydrogen atmosphere (with a scan rate of 50 mV/s)
[9].

Figure 3.PEC measurements on ATO and ATO-H-10. (a) LSV curves of ATO-H-10 photoanode as a function of scan rates in 1 M KOH under simulated
solar illumination. (b) LSV curves of pristine ATO and ATO-H-10 with a scan rate of 5 mV/s under simulated
solar illumination. (c) IPCE spectra of pristine ATO and ATO-H-10 in the range of 300 to 700 nm at 0 V (vs
Ag/AgCl). Inset: magnified IPCE spectra, highlighted in dashed box, at the incident
wavelength range of 430 to 700 nm.

The STH efficiency (η) on the photoanodes is calculated using the following equation
[28]:

where V is the applied bias voltage vs reversible hydrogen electrode (RHE), I is the photocurrent density at the measured bias, and Jlight is the irradiance intensity of 100 mW/cm2. The pristine ATO exhibits a STH efficiency of 0.19% at -0.64 V (vs Ag/AgCl), while
the ATO-H electrode yields a much improved efficiency (η = 0.30%) at -0.48 V (vs Ag/AgCl). Moreover, the quartz window reflects more than
4% of the solar irradiance
[29], which means that the internal STH efficiencies are higher than the calculated values.
Using front-side illumination configuration could reduce this loss and further boost
the conversion efficiency
[9].

IPCE measurements are carried out to investigate the contribution of each monochromatic
light to the photocurrent density. Compared with the measurements based on the wide
band light source without taking into account the differences between the spectra
of the light source and the solar spectrum, and/or reliable calibration, which may
vary from different research laboratories, the intensity-independent IPCE provides
a reliable method to characterize the wavelength dependent photoresponse. The IPCE
is calculated as a function of wavelength using IPCE = (1,240 (mW⋅nm/mA)I) / (λJlight), where λ is the incident light wavelength (nm) and I and Jlight are the photocurrent density (mA/cm2) and incident light irradiance (mW/cm2) at a specific wavelength
[28].

Figure
3c shows the IPCE plots of ATO and ATO-H-10 at zero bias vs Ag/AgCl. The results indicate
that the enhanced photocurrent is mainly contributed by UV response due to electrical
conductivity modification. Reductive doping gives rise to a pronounced enhancement
in full UV region (300 to 400 nm) with a maximum value of 82% at 360 nm. The decrease
at shorter wavelengths could be attributed to the unwanted light reflection or absorption
before arriving to a photoanode
[29]. In the longer wavelength region, IPCE plots represent abrupt decreases from approximately
49% (ATO) and approximately 74% (ATO-H-10) at 370 nm to less than 2% at 410 nm, which
is determined by the recombination of charge carriers in the wide bandgap (approximately
3.2 eV) anatase TiO2[30]. A weak photoactivity of pristine ATO nanotube in 400 to 600 nm could be ascribed
to fluorine doping during anodization in NH4F-containing electrolytes
[9,31]. In addition, a slightly enhanced photocurrent can also be observed in the visible
range (410 to 600 nm) on ATO-H-10 electrode (inset of Figure
3c). The oxygen vacancy states are generally localized with energies of 0.75 to 1.18
eV below the conduction band, which is lower than the redox potential for hydrogen
evolution
[32,33], while a high vacancy concentration could produce shallow donor levels just below
the conduction band, which in turn provides enough energy for water splitting
[34]. The experimental results suggest the formation of shallow levels which is responsible
for the slightly enhanced visible light activity.

Further insight into the TiO2 characteristics is conducted by electrochemical impedance spectroscopy (EIS) measurements
in the frequency range of 0.01 Hz to 100 kHz. Figure
4a shows the Nyquist plot of ATO and ATO-H-10 electrodes in dark condition. The intercepts
of both plots on the real axis is less than 4 Ω, representing the conductivity of
the electrolyte (Rs). In contrast with the large semicircle diameter of pristine ATO electrode, an extremely
small semicircle diameter for ATO-H-10 electrode (inset of Figure
4a) indicates a much improved electrode conductivity with significantly low charge
transfer resistance
[35].

It is known that PEC performance of the electrode is determined by charge separation
and transfer process. Besides offering increased donor states, the introduced defect
states would also serve as recombination centers for electron–hole pairs and consequently
inhibit the charge collection. The visible luminescence band of anatase TiO2 is caused by donor-acceptor recombination, which is closely related to both trapped
electrons and trapped holes
[36]. In the nanocrystalline electrode, photoexcited carriers are readily captured in
the inherent trap states. Trapping and thermally detrapping mechanisms will determine
the slow decay process
[37]. It is believed that the inherent shallow trap states in pristine ATO, serving as
electron trapping sites, mainly contribute to the slow decay process. Subsequently,
electrochemical hydrogenation could introduce more defect states into shallow energy
levels to capture excited electrons, which will prolong the relaxation processes with
the corresponding longer lifetime. The dynamic characteristics of photogenerated carriers
are revealed by room-temperature TRPL spectroscopy. Figure
4b displays the TRPL curves of the different electrodes recorded at 413 nm with a 375-nm
pulsed laser as excitation source. The ATO-H-10 electrode shows a somewhat longer
lifetime compared with the pristine ATO electrode. This means that the electrochemical
reductive process is an efficient strategy to improve electrode conductivity, with
a slightly increased recombination rates. By utilizing single exponential decay fitting
on the obtained curves, the averaged photoluminescence lifetimes of ATO and ATO-H-10
are calculated to be 537 and 618 ps, respectively.

Conclusions

In conclusion, the electrochemical reductive doping processes are carried out to produce
hydrogenated ATO photoanodes to improve PEC water splitting efficiency. A -5-V bias
voltage, with only 10 s of processing time, yields a substantially enhanced photocurrent
density of 0.29 to 0.65 mA/cm2. IPCE results indicate that the enhanced STH efficiency in ATO-H-10 is dominantly
contributed by the improved photoactivities in the UV region. The electrochemically
induced oxygen vacancies lead to increased donor density, which is responsible for
the enhanced photocurrent with slightly increased parasitic recombination. This eco-friendly
approach opens up a novel strategy for significantly improving the photoanode performance
and provides potential for large-scale productions.

Competing interests

The authors declare that they have no competing interests.

Author’s contributions

XYC, XFZ, and DDL designed the experiments. CX, XHF, and LFL carried out the experiments.
CX, YS, CWC, and DFL performed electrode characterization and data analysis. CX and
DDL wrote the paper. All authors read and approved the final manuscript.

Acknowledgements

We thank Professor Xiangyang Kong for his helpful discussions and technical assistance.
This work is financially supported by the National Natural Science Foundation of China
(grant nos. 61171043, 51077072, 11174308 and 51102271), Shell Global Solutions International
B.V. (PT31045), the Natural Science Foundation of Shanghai (11ZR1436300), and the
Shanghai Municipal Human Resources and Social Security Bureau (2011033).

References

Fujishima A, Honda K: Electrochemical photolysis of water at a semiconductor electrode.